In designing magnetic disk memory drives for computers, one wishes to maximize both the accessible information density of the disk and the life span of the drive. In most disk drives, access to the disk is via a read/write head that is located at the end of an actuator arm. The disk itself is usually divided into a series of concentric "tracks," and the disk rotates under the head. Each track, in turn, is divided into a series of adjacent data areas of minimum size, and each data area contains one "bit" of magnetically encoded information. U.S. Pat. No. 4,414,594 (Farmer, Nov. 8, 1983) illustrates a conventional disk drive system which uses a linear actuator to position read/write heads.
Obviously, the more "bits" one can pack into each track, and the more tracks one can fit on a disk, the more data can theoretically be stored on the disk. Information density, often measured in terms of "bits per inch" (BPI, measured linearly per track) and "tracks per inch" (TPI), therefore depends in part on how finely one can subdivide the surface of the disk into tracks and bit areas.
Even if one greatly increases the density of the disk, however, it may still be impossible to read the disk reliably because of the mechanical limitations of the drive. A major limiting factor in increasing the useful memory capacity of a disk drive is known as "non-repeatable run-out error," which is caused by an inability to locate the read/write head with sufficient precision and random radial movement of the disks caused by imperfections in the spindle motor bearings. Actuator arms in conventional disk drives require some bearing around which they rotate, moving the read/write head near the end of the arm toward and away from the center of the disk and holding the head immediately above the surface of the disk.
Non-repeatable error from the head actuation system arises because of random "wobbling" of the arm on its bearing, that is, the axis of rotation of the arm shifts slightly because of imperfections in the bearing. As the arm wobbles, the head will also move, but since the amount and direction of movement of the axis of rotation of the arm are random, it would be impossible to know for sure which bit area the head is over if the head moves randomly over more than one bit area. By way of example, assume that a particular disk has 1000 tracks per inch (TPI), that is, that each of the concentric tracks of the disk is approximately 0.001" wide (this is a typical value for track spacing on existing disks.) If the bearing radial movement is such that one cannot know the exact position of the head to better than .+-.0.0004" (two-fifths the track width), it would not be possible to use higher-density, 4000 TPI disks: since the track spacing on 4000 TPI disks is 1/4000"=0.00025", and since this is less than the "play" of the head, one could never be certain that the head was now reading a particular track and not an adjacent one.
A common way to decrease bearing shift is to pre-load the bearings, which are often high-precision needle or ball bearings. For a while, this lessens shift since the actuator arm is joined more "tightly" to the pivot shaft. This solution, however, leads to quicker deterioration of the bearing itself due to the increased wear caused by greater pre-loading. As the bearing wears down, gaps once again arise and widen, and the problem of bearing shift returns, bringing with it the very problem of non-repeatable error pre-loading was meant to cure. Pre-loading therefore trades life span for precision.
Pre-loading is not the only force acting to wear down the pivot bearing of the actuator arm. In order to move the arm, some form of actuator motor is used to torque the arm about its pivot bearing. The greater the power of the motor, the faster the arm can accelerate and change tracks on the disk. Commonly used rotary actuators apply a magnetic force to the end of the arm opposite the head. The applied force, however, leads to a reaction force on the bearing. This reaction force, like the force of pre-loading, in turn causes wear on the bearing and leads sooner or later to greater bearing radial movement and to greater non-repeatable error. Using conventional technology, therefore, increased reading and writing speed for the head also comes at a cost of decreased long-term reliability and life span for the disk drive.
Mechanical resonance is another major problem that affects all disk drives. Historically, all hard disk drives have, to a greater or lesser degree, experienced performance problems related to resonant frequencies. Such resonant frequencies, if excited by drive operation or environmental conditions such as vibration, can significantly reduce the ability of the drive to keep the read-write head at the centermost part of the track width. This is known to those skilled in the art as "track following error". In worst cases the actuator may exhibit a radial displacement due to resonance which is a significant percentage of the track width and can lead to a read or write failure. As track width decreases to attain a greater number of tracks per inch, TPI, this problem becomes of greater importance.
Until recently, most resonance problems were related to resonant frequencies which were less than 3000 Hz (3 KHz). The stiffness of the assembly was generally increased to cause these resonances to then appear above 3 KHz; this was sufficiently high to allow acceptable performance for the TPI required at the time. This increased stiffness often included increasing the bearing preload.
Often the mass of the moving structures was decreased to increase the resonant frequency of that structure. As disk drives became smaller, the mass of the moving parts decreased accordingly and the resonant frequencies of the various modes of vibration moved even higher, in general, to above 5 KHz. This again was sufficiently high to allow acceptable performance for the new, higher TPI required for the new, smaller drives.
As hard disk drives become even smaller, the pivot bearing structure becomes a significant part of the moment of inertia of the rotating parts. However, it is not practical to use a smaller ball bearing because of the reaction forces this bearing must withstand.
Also, as hard disk drives have become smaller, another problem has arisen in the form of a low-frequency resonance. This problem is most apparent during track-following operations. With narrow track widths and the associated very small motions required to cause the head to remain near the center of the track, the pivot ball bearing structure does not immediately rotate in response to the force of the actuator, but reacts in a manner similar to a torsion spring, which has a variable spring rate. It is this spring-like behavior that gives rise to the low-frequency resonance that unfortunately occurs at or just above the disk rotational frequency. This resonance thus is easily excited by any disk imbalance or periodic spindle motor torque variation.
Any attempt to decrease the bearing preload to lower this resonant frequency below the disk rotational frequency results in a corresponding decrease in the higher resonant frequencies, thereby dropping them into dangerous frequency ranges incompatible with high TPI requirements. Furthermore, such reduction in bearing stiffness may cause head mis-position due to the reaction forces on the bearing, which are caused by the traditional actuator forces.
Again it is apparent that a reduction in reaction forces at the pivot bearing allows the designer to consider smaller or different bearing structures that can reduce the low-frequency resonant frequencies below the disk rotational frequency and yet allow the high- frequency resonances to stay high enough to accommodate the required high TPI.
Another theoretically possible way of reducing the load on the bearing would be to mount the arm directly onto the rotor shaft of a rotary motor. In so doing, pure torque would be applied to the arm and no significant reaction force would result on the shaft or its bearing. A rotary actuator motor, however, suffers from several drawbacks. First, since the moment arm for torquing the arm is short, the motor must be more powerful to achieve the same acceleration of the arm as compared to an actuator that acts on the end of the arm.
Second, rotary motors generate relatively strong magnetic fields, which often tend to disturb the reading and writing of the magnetically encoded data on the disk, or which require special shielding to prevent such disturbance. (Even non-rotary motors according to the prior art often suffer from this disadvantage.) Furthermore, the inductance of conventional rotary motors is so great that it takes a relatively long time for them to switch polarity and reverse head direction. In the case of a solution based on a rotary motor, therefore, longer bearing life comes at the cost of speed: one may perhaps be able to increase bit density, but it will take longer to read or write data to the disk.
Yet another disadvantage of many existing designs is that the actuator motors require so much vertical space that they are unsuitable for use in small or thin disk drives.
Still another shortcoming of many existing actuator motors is that the torque they apply to the arm varies depending on the angular position of the arm. This means that the acceleration of the arm is less when the head is over certain tracks of the disk than when it is over other tracks. Because of this, the time it takes for the arm to move between two tracks will vary depending not only on the relative distance between the tracks, but also on where on the disk the tracks happen to be. Access time for data over the surface of the disk is therefore not uniform.
In order to deal with the problem of non-uniform torque, some actuators use two or more coils whose accumulated forces are supposed to remain constant even though each coil's generated force varies. In other actuators, the cross-sectional shape of each coil is designed in such a way that, as the permanent magnets mounted on the arm move relative to the coils, the decrease in generated force is compensated for by an increased moment arm, so that, at least in theory, the product, and thus the torque, remains constant; in still other actuators, the same effect is achieved with one or more movable coils and fixed permanent magnets.
The problems with these solutions arise both at the manufacturing stage and during actual operation. First, increasing the number of coils usually increases mass and inductance, and makes such actuators ill-suited for use in compact disk drives where the magnetic memory medium is close to the actuator. Second, the relationship between the shape of the permanent magnets and the coils is limited by the necessity of keeping the force/moment arm product constant; this, in turn, increases design complexity and limits design flexibility.
Mounting permanent magnets on the arm itself not only increases the mass of the arm and slows its acceleration, but it also makes it more difficult to contain the magnetic field and prevent leakage to the disk or other magnetic memory medium and possible destruction of data. Furthermore, even though one may attempt to design the magnets and coils to maintain a constant torque regardless of their relative position, in practice, since the lines of magnetic flux are seldom perfectly straight, with easily calculated density, it is not possible to achieve truly constant torque.